1. Field of the Invention
The present invention relates to shape memory polymers and, more specifically, to a shape memory polymer having a range of transition temperatures that are spatially distributed in a gradient fashion within one single article.
2. Description of the Related Art
Shape memory polymers (SMPs) are a class of “smart” materials that can switch between two shapes on command, from a fixed (temporary) shape to a pre-determined permanent shape upon the application of an external stimulus such as heat. This shape memory behavior is generally characterized using programmed, cyclic thermomechanical tests referred to as the shape memory cycle (SMC). In a typical SMC, the SMP is first deformed at an elevated temperature that is higher than its transition temperature, Ttrans (either Tm or Tg). The deformation is elastic in nature and mainly leads to a reduction in conformational entropy of the constituent network chains, following the rubber elasticity theory. Commonly applied deformation modes include tension, compression, and bending. The deformed SMP is then cooled to a temperature below its Ttrans while maintaining constant the external strain or stress. During cooling, the material transitions to a more rigid state (semi-crystalline or glassy), which kinetically traps or “freezes” the constituent network chains in this low-entropy state. Macroscopically the material retains, or “fixes,” the temporary strain/shape even when external stress is released. Shape recovery is finally triggered by heating the material through Ttrans under a stress-free (unconstrained)—or even loaded (constrained)—condition. By allowing the network chains (with regained mobility) to relax to their thermodynamically favored, maximal-entropy state, the material changes from the temporary to its permanent shape. Two characteristic ratios, fixing ratio (Rf) and recovery ratio (Rr) characterize the shape memory performance (shape fixing and shape recovery) for comparison among different material systems.
SMPs have several intrinsic advantages over the traditionally used shape memory alloys (SMAs) including larger deformation strains, tunable transition temperatures, low density and low manufacturing cost. As a result they have attracted a significant amount of research interest during the past decade. Novel SMPs have been developed with responsiveness to non-heat stimuli such as light, electricity, and magnetic field, and with new recovery behavior including two-way shape memory and triple-shape memory.
The stimuli-responsiveness gives SMPs an ability to sense environmental changes such as an increase of temperature, and respond in a prescribed manner. However, the application of conventional SMPs as temperature sensors is still limited, mainly due to the fact that there is usually only one Ttrans associated with a given material, as determined by its constituent molecular composition and architecture. In other words, conventional SMPs only respond to a threshold temperature trigger and are unable to respond to temperatures over a broad range.
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Description Of the Related Art Section Disclaimer: To the extent that specific publications are discussed above in this Description of the Related Art Section, or elsewhere herein, these discussions should not be taken as an admission that the discussed publications (for example, technical/scientific publications) are prior art for patent law purposes. For example, some or all of the discussed publications may not be sufficiently early in time, may not reflect subject matter developed early enough in time and/or may not be sufficiently enabling so as to amount to prior art for patent law purposes. To the extent that specific publications are discussed above in this Description of the Related Art Section, or elsewhere herein, they are all hereby incorporated by reference into this document in their respective entirety(ies).